2000 Hook-up Book - Spirax Sarco

SYSTEM DESIGN
56
Testing Steam Traps
What must be done, using all
audible and visual clues, is to
detect normal or abnormal
cycling of the discharge. Even
this method is very fallible, since
the mode of operation of different
trap types if not nearly so well
defined as is sometimes thought.
Table 16 lists some of the possibilities
and allows the problem to
be seen more clearly.
It is seen that the “signal” to be
obtained from the trap, whether
visual, audio or temperature, is
usually going to be so ambiguous
as to rely largely on optimism for
interpretation. The one trap which
is fairly positive in its action is the
disc thermodynamic type—if this
is heard or seen to cycle up to ten
times per minute, it is operating
normally. The cycling rate increases
when the trap becomes worn
and the characteristic “machine
gun” sound clearly indicates the
need for remedial action.
Spira-tec Leak Detector
System
Logic says that if it is not possible
to have a universally applicable
method of checking steam traps
by examining the traps themselves,
then we must see if it can
be done by checking elsewhere.
This is what Spirax Sarco has
done with the Spira-tec system.
See Fig. 61 (page 58).
The Spira-tec detector chamber
is fitted into the condensate
Table 16: Steam Trap Discharge Modes
Mode of Operation
Full or Usual
Trap Type No Load Light Load Normal Load Overload Failure Mode
Float & Usually continuous but may Closed,
Thermosatic No Action cycle at high pressure Continuous A.V. Open
Inverted Bucket
Balanced Pressure
Small Dribble Intermittent Intermittent Continuous Open
Thermostatic No Action May Dribble Intermittent Continuous Variable
Bimetallic Usually Dribble May blast at
Thermostatic No Action Action high pressures Continuous Open
Impulse
Disc
Small Dribble
Usually continuous
with blast at high loads Continuous Open
Thermo-Dynamic No Action Intermittent Intermittent Continuous Open
line on the inlet side of the trap. If
there is, at this point, a normal flow
of condensate towards the trap,
together with a small amount of air
and the steam needed to make up
heat loss from the body of the
steam trap, then all is normal. On
the other hand, an increased flow
of gas along the pipe indicates
that the trap is leaking.
The chamber contains an
inverted weir. Condensate flows
under this weir and a small hole
at the top equalizes the pressure
on each side when the steam trap
is working normally. An electrode
on the upstream side of the baffle
detects the presence of condensate
by its conductivity which is
much higher than that of steam.
By plugging in the portable indicator,
it is possible to check if the
electrical circuit is complete when
a visual signal indicates that the
trap is working.
If the trap begins to leak
steam, then the pressure on the
downstream side of the weir
begins to fall. The higher pressure
on the upstream side drops the
condensate level below the electrode
and exposes it to steam.
The “conductivity” circuit is broken
and the indicator light gives a
“fail” signal.
The advantage of the system
lies in the very positive signal
which does not require experience
of personal judgement
before it can be interpreted.
Using suitable wiring, the test
point can be located remote from
the sensor chamber or it can
have a multi switch to allow up to
twelve (12) chambers to be
checked from a single test location.
When appropriate, an
electronic continuous 16-way
checking instrument can monitor
the chambers and this is readily
connected into a central Energy
Management System.
The object of detecting leaking
steam traps is to correct the
problem. This can mean replacement
of the whole trap, or
perhaps of the faulty part of the
internal mechanism. It is very
useful indeed to be able to check
a repaired trap in the workshop
before it is installed in the line,
and many repair shops now use a
Spira-tec chamber as part of a
bench test rig. The diagram
shows a simple hookup which
allows suspect or repaired traps
to be positively checked. (Fig. 60)
Cost Of Steam Leaks
The installation and use of the
Spira-tec units does involve some
cost, and it is necessary to compare
this with the cost of steam
leakages to see if the expenditure
is economically justifiable. Since
all equipment must wear and
eventually fail, we need first an
estimate of the average life of a
steam trap. Let us assume that in
a particular installation, this is,